Magnetoresistive random access memory (MRAM) utilizes magnetic tunnel junctions (MTJs) to store digital information. FIG. 1, for example, shows a typical MTJ 100. The MTJ is formed on a substrate 110 and comprises a bottom metal layer 120, a dielectric layer 130 and a top metal layer 140. Each of the metal layers is ferromagnetic and has an associated magnetic orientation. The bottom metal layer has a magnetic orientation which is fixed in given direction while the top metal layer is allowed to switch between two preferred orientations when exposed to an applied magnetic field. Writing digital information to the MTJ comprises applying a magnetic field to the MTJ in order to switch the magnetic orientation of the top metal layer between these two preferred directions.
In order to read the MTJ 100, electrons are made to tunnel through the dielectric layer 130 by applying a voltage to the MTJ and determining the electrical resistance of the dielectric layer. The resistance of the dielectric layer, in turn, depends on the relative magnetic orientations of the adjacent metal layers 120, 140. The resistance of the dielectric layer is relatively low when the magnetic orientations of the metal layers point in substantially the same direction. In contrast, the resistance of the dielectric layer is relatively high when the magnetic orientations of the metal layers point in opposite directions. The relative change in resistance is termed magnetoresistance (MR), and is expressed as a percentage change with respect to the lower resistance value.
It is often advantageous for a design or process engineer to electrically characterize an MTJ film stack in wafer form before the film stack is further processed into discrete MTJ devices like the MTJ 100. Conventional techniques for characterizing MTJ film stacks in wafer form, however, typically require extensive processing of the semiconductor wafer in order to measure a few characteristics of the MTJ film stack. For instance, it is often necessary to attach external leads to various metal layers in the MTJ film stack. This processing is time consuming, complex and can ruin devices. In addition, even if the additional processing creates suitable devices for test, it is frequently unclear whether the resultant measurements have been influenced by the added processing and structures. In other words, it is unclear as to whether the measurements are a function of the MTJ film stack itself, the processing that creates additional structures needed to make the measurements, the additional structures, or some combination thereof.
Fortunately, U.S. Pat. No. 6,927,569, co-invented by the inventor of the present invention and entitled “Techniques for Electrically Characterizing Tunnel Junction Film Stacks with Little or No Processing,” provides methods for characterizing some kinds of MTJ film stacks without the need for extensive additional processing. These techniques utilize, in part, resistance measurements made via a multi-point probe apparatus capable of making voltage and current measurements with variable probe spacings. Advantageously, the sheet resistances of various constituent metal layers and the MR of the MTJ film stack's dielectric layer can be readily determined by these techniques.
Nevertheless, recently, it has become increasingly more popular to form MTJs with more than one dielectric layer. FIG. 2, for example, shows an illustration of a two-dielectric-layer MTJ 200. This MTJ is formed on a substrate 210 and comprises a bottom metal layer 220, a bottom dielectric layer 230, a middle metal layer 240, a top dielectric layer 250 and a top metal layer 260. As is typical in these types of devices, the top dielectric layer serves as a barrier for the interdiffusion of metallic elements while the bottom dielectric layer acts as the barrier layer wherein the magnetoresistance effects occur. It is, therefore, desirable to determine the MR for this bottom dielectric layer. Unfortunately, because of the presence of the second dielectric layer, the methods provided in the above-cited patent cannot be used. Consequently, the need exists for new techniques and apparatus that allow an MTJ film stack with two dielectric layers to be electrically characterized without requiring additional processing.